1. Field of the Invention
This invention relates to compact zoom lenses. Although the invention has general application, it is particularly usable in still photographic cameras.
2. Background Art
U.S. Pat. No. 5,270,867 to Lee R. Estelle, issued Dec. 14, 1993, described zoom lenses (also called zoom lens systems) having two units of lens components--a positive front unit and a negative rear unit. The disclosed zoom lenses utilize only three or four lens components and still achieve very good aberration correction for zoom ranges 1:2 and aperture ratios of f/8 to f/11.
U.S. Pat. No. 4,936,661 to E. I. Betensky et al., issued Jun. 26, 1990, described a zoom lens with a short back focal length and having, from front to rear, negative, positive and negative optical units. The negative unit closest to the image is movable during zooming to provide a majority of the change in focal length. In some of the examples, the front two units move as a single optical group during zooming, and in others they move relative to each other during zooming. These lenses have a short back focal length which makes them particularly useful as zoom objectives in "viewfinder" (non-SLR) cameras.
Both of the above-mentioned patent applications disclose well corrected 2.times. zoom lenses. However, there is an increasing need to provide consumers with a well corrected zoom lens comprised of only three to four lens elements which has an expanded zoom range such as 3.times. or higher. In addition, many zoom lenses of the type described above utilize aspheric surfaces. These aspheric surfaces are generally sensitive to decentration and when decentered, they usually produce image plane tilt. Thus, there is also a need for zoom lens systems with an expanded zoom range which have reduced aspheric decenter sensitivity.